A turbocharged engine may include high pressure exhaust gas recirculation (EGR) and low pressure EGR. High pressure EGR may be provided to an engine by passing exhaust gas from an exhaust system at a location upstream of a turbocharger turbine to an engine intake system at a location downstream of a turbocharger compressor. Low pressure EGR may be provided to an engine by passing exhaust gas from an engine exhaust system at a location downstream of a turbocharger turbine to an engine intake system at a location upstream of the turbocharger compressor. Low pressure EGR may have the benefit of being cooler than high pressure EGR so that engine charge temperature may be reduced. On the other hand, by using high pressure EGR, an engine control system may reduce an EGR mass fraction inducted into a cylinder at a faster rate in response to a change in engine load as compared to when low pressure EGR is provided to the engine. Thus, there may be advantages and disadvantages to using high pressure EGR and low pressure EGR.
The inventors herein have also recognized that high pressure EGR and low pressure EGR may be comprised of the same or different constituents. Consequently, engine emissions may vary depending on whether high pressure EGR or low pressure EGR is supplied to the engine. The inventors have addressed the differences between supplying high pressure EGR and low pressure EGR to an engine by developing a method for operating an engine, comprising: adjusting an actuator in response to a NOx mass flow rate in a low pressure EGR passage between an engine exhaust system and an engine air intake system.
By adjusting an actuator responsive to a NOx mass flow rate in a low pressure EGR passage, it may be possible to provide a technical result of adjusting engine NOx emissions to a desirable level. For example, if EGR is being supplied to the engine with a low NOx mass flow rate, an engine actuator may be adjusted to increase the engine's NOx mass flow output and engine fuel economy such that engine's NOx mass flow output remains below a threshold NOx level. Alternatively, if EGR is supplied to the engine with a higher NOx mass flow rate, the engine actuator may be adjusted to decrease the engine's NOx mass flow output. NOx supplied to the engine via EGR passes through the engine and cannot be reduced during combustion via adjusting engine operation. However, NOx formed during combustion of an air-fuel mixture may be adjusted inversely with respect to NOx supplied to the engine via EGR so that a desired engine NOx level may be provided. Thus, if the desired engine NOx mass flow rate is a constant, and if the NOx flow rate of exhaust gases located downstream of a selective catalytic reduction (SCR) catalyst is decreasing because of higher SCR efficiency, NOx formed in the engine as a result of combustion may be increased without increasing the engine's NOx mass flow rate since NOx flowing into the engine via EGR is decreasing.
The present description may provide several advantages. For example, the approach may allow engine emissions to be maintained at a desired level while engine fuel economy is improved. Additionally, the approach may be useful for improving the exchange of urea use for vehicle fuel economy. Further, the approach may be useful for improving transient engine emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.